Answer:
Since the equation is addition, reverse it into subtraction. 210-53.715-51.3-52.62= 52.365. You can add 53.715 by 51.3 by 52.62 by 52.365 to check your answer, and it will be 210 so the 4th runner ran 52.365 seconds.
i also got this question but i figured it out lol